1. Field of the Invention
The present invention relates to a planar light source device making the rear face of a light guide member have a light scattering function and uniformly outputting light to the outside, and an image reading device utilizing the same planar light source device.
2. Related Art
Up to now, as a planar light source device there has been known for example a light source device disclosed in Japanese Patent Laid-Open Publication No. 2001-34,210. A planar light source device disclosed in this patent publication is shown in FIG. 1. The planar light source device shown in FIG. 1 comprises a planar light guide member 12 made of transparent acrylic resin, and this planar light guide member 12 comprises LED light sources 14a and 14b respectively in the middle of the short sides being opposite to each other. And light scattering members are formed all over the rear face 13 of the planar light guide member 12. The bottom face of the planar light guide member 12 is covered with a white bottom plate 16, and four side faces are covered with a white case frame 18. A diffuser sheet 20 is adhered to the top face (light emitting face) of the planar light guide member 12.
This planar light source device uses a mechanism making the light from a point light source enter a planar light guide member made of a transparent material and uniformly emitting light from its two-dimensional planar portion in order to convert the light from an LED light source to be considered as a point light source into distributed light from a planar light source.
In addition, an arrangement having one point light source on each of both sides opposite to each other of a planar light guide member makes an in-surface luminance distribution provided by the quantity of light of one light source and an in-surface luminance distribution provided by the quantity of light of the other light source compensate each other and finally obtains a uniform luminance distribution with the in-surface luminance distribution obtained by adding both the in-surface luminance distributions to each other.
However, when attempting to increase the number of colors or the quantity of light by increasing the number of LED light sources in a planar light source device as described above, the following problems occur.
In order to uniformly emit light from a two-dimensional planar portion by converting the light from a point light source into the distributed light from a planar light source, it is necessary to form light scattering members very accurately adjusted on the rear face of a planar light guide member. However, light scattering members formed on the rear face of a planar light guide member are ordinarily adjusted so as to uniformly scatter only the light from a single LED module (light emitted from nearly one point). The adjustment for coping with light emitted from one point (a point light source) makes the adjustment of light out of order in relation to a new point light source newly provided adjacently to the said one point light source, and as a result the distribution of luminance of a planar light source is made irregular, and therefore it is difficult to mount a plurality of LED modules at some intervals.
Accordingly, the number of LED modules to be mounted on a planar light guide member is limited to one by such restrictions as described above, ordinarily, a single LED module (for example, Model NSCM315C made by Nichia Chemical Industrial Co.) has three LED chips of R (red), G (green) and B (blue) mounted on it. Mounting more chips than this causes a heat generating element to be confined in a substrate limited in area, makes the whole substrate rise in temperature due to an insufficient cooling action, and degrades the LED's in optical performance. Accordingly, for example, an LED module mounted with LED's of three kinds of R, G and B cannot be ordinarily mounted with chips other than them at the same time, and therefore results in becoming a planar light source having chips limited (in number or color). That is to say, it is difficult to make a light source multicolor or increase the quantity of light.
Here, in case of providing light sources on both face sides of a planar light guide member being opposite to each other, only the same kind of light sources can make a luminance distribution uniform as a whole. Only arranging the same kind of point light sources opposite to each other provides a planar light emission being uniform due to integration of light quantity, and arranging different kinds of point light sources opposite to each other results in making an abnormal in-surface distribution of light quantity due to no integration.
Further, in case of LED light sources, even LED's of the same kind have variations in light quantity. In case of LED light sources, even products of the same kind ordinarily have large variations in light quantity and have a maximum-to-minimum ratio of 3 to 4 in optical output in their specifications, and the combination of LED's being different in optical output cannot bring a uniform distribution of light quantity due to unbalanced weighting in integration.
The present invention has been performed in order to solve such conventional problems and an object of it is to provide a planar light source device making it possible to use LED light sources of many kinds (many colors), many LED light sources of the same color (increased in light quantity) or combination of them by arranging a plurality of LED light sources on an end face of a light guide member, and an image reading device utilizing the same planar light source device.